The present invention relates to the art of synchronous motor speed control. It finds particular application in conjunction with speed control circuits for rotating anode x-ray tubes and will be described with particular reference thereto. However, it is to be appreciated that the invention may find application in conjunction with other speed control circuits, particularly those in which the controlled motor has low efficiency.
Heretofore, rotating anode x-ray tubes have included a sealed and evacuated envelope in which the cathode, anode, anode bearings, anode rotor, and other associated structures were sealed. Stator windings were mounted outside of the envelope adjacent the rotor. In this manner, a synchronous motor, particularly a two-phase squirrel cage type of motor was created.
Commonly, these motors ran about one or two RPM slower than the frequency of the current applied to the stator. In countries, such as the United States, in which the line frequency is 60 Hz, the anode was rotated at about 58-59 RPM. Analogously, in Canada and countries that utilize a 50 Hz line frequency, the rotor was rotated at about 48-49 RPM.
One problem encountered with rotating anode x-ray tubes is that 4" rotors have a natural resonance frequency around 60 Hz and 5" anodes have a natural resonance frequency around 50 Hz. At the resonance frequency, the anodes tend to vibrate. Such vibration, of course, causes bearing wear and premature bearing failure. Perhaps more significantly, the vibrations of the anode cause corresponding fluctuations in the x-ray output of the tube. These x-ray fluctuations cause as "rotor ripple" artifacts in x-ray diagnostic images.
These x-ray fluctuations cause degradation in many x-ray tube applications. For example, in medical diagnostic imaging, these fluctuations in the x-ray intensity compromise the diagnostic value of the resultant diagnostic images.
One solution is to change the frequency of the line current applied to the stator windings. However, such frequency altering circuitry is relatively expensive.
In accordance with the present invention, a technique is provided for economically shifting the rotational speed of the rotating anode away from its resonant frequency.